I need help debugging this script, It only works in my studio, not in servers?

It's because all the variables you are declaring are not in the player's gui environment and mabye it has something to do with filtering enabled.

It should be:

local player = game.Players.LocalPlayer

local barhold = script.Parent.loadingFrame -- now declare your variables like player.PlayerGui.YourScreenGui.YourLoadingFrame
local bar = barhold.loadingBar
local screen = script.Parent
local txt = script.Parent.Text
local info = script.Parent:WaitForChild("info")
local loaded = 0
_G.soundassets = {164508682, 176135621, 200907510, 142402325, 190547014, 197321696, 165937331, 198507773, 163611921, 165065112, 187768662, 243171299, 243171064, 243170995, 243170853, 173607522, 173607581, 173607688, 173607557, 173600950, }
cp = game:GetService("ContentProvider")
sss = game:GetService("ServerScriptService")

ui = require(sss:WaitForChild('UIManipulator'))
ui.Fade(barhold, "In", .1)
ui.Fade(bar, "In", .1)

info.Visible = true

for _, AssetId in pairs(_G.soundassets) do
        cp:Preload("http://www.roblox.com/asset/?id=" .. AssetId)
        loaded = loaded +1
        info.Text = "Loading asset: "..AssetId.." || "..loaded.." Out of "..#_G.soundassets
        bar:TweenSize(UDim2.new(loaded/#_G.soundassets, 0, 1, 0), Enum.EasingDirection.InOut, Enum.EasingStyle.Quint)
        wait(1)
end

info.Visible = false

bar:TweenSize(UDim2.new(0,0,1,0), Enum.EasingDirection.InOut, Enum.EasingStyle.Quint, 1)
wait(1)
ui.Fade(bar, "Out", .1)

ui.Fade(barhold, "Out", .1)

ui.Fade(screen, "Out", .1)
screen.Parent:Destroy()